1. Field of the Invention
This invention relates to a load driving system and method thereof for driving loads using a power source having an output voltage higher than a normal driving voltage of the loads, and more particularly, to an art of simplifying the circuitry and reducing its heat generating amount.
2. Description of the Related Art
The voltage of a battery power source mounted to vehicles such as automobiles or trucks is generally 12 volts or 24 volts. The recent trend is to use a higher battery voltage (52 volts, for example) in order to reduce the value of the current flowing through various circuits or loads mounted to a vehicle. However, such loads as lamps mounted to a vehicle are designed for a conventional battery voltage of 12 volts or 24 volts, and cannot be driven when being directly connected to a 52-volts DC power source.
It is thus necessary to replace such loads mounted to a vehicle with ones designed for a voltage of 52 volts to match 52-volts battery voltage. However, such increase in a driving voltage of such lamps as head lamps, tail lamps, brake lamps and room lamps mounted to a vehicle (that is, designing the lamps for 52 volts) results in increase in the size of these devices, causing cost increase, and thus being impractical.
Under these circumstances, various kinds of load driving system for driving loads with a voltage higher than a normal driving voltage without undue stress have been proposed and put to practical use. An example of such conventional load driving systems is disclosed in Japanese Patent Laid-Open Publication No. Hei-5-168164 (hereinafter referred to as a xe2x80x9cconventional examplexe2x80x9d).
FIG. 1 is a circuit diagram of a load driving system described in the conventional example. As shown in the figure, the load driving system has a DC power source 101 and an FET 106 directly connected to a lamp 102 as a load. The system further includes a voltage detector 104, a PWM controller 105, and a switch 103.
The PWM controller 105 outputs a pulse signal to the FET 106 to cause it to turn on and off so as to make a root-mean-square value of the voltage supplied to the lamp 102 smaller than the output voltage of the DC power source 101, thereby preventing the lamp 102 from being overloaded when illuminated.
However, since the above conventional load driving system uses the DC power source 101 having a voltage higher than the normal driving voltage of the load, the turn-on of the power causes an inrush current flow excessively greater than that in the normal driving voltage operation. This is conspicuous when the load is a lamp. The generation of inrush current causes such a problem as a shortened life of the load.
Accordingly, even though the conventional load driving system can control the turn on and off of the FET 106 by PWM control to drive a load for 12 volts connected to the DC power source 101 outputting a voltage of 52 volts, for example, an excessive inrush current flows at power-on, applying great load to the circuitry, and shortening the life of the load.
This invention was made to solve the above conventional problems, and to provide a load driving system capable of limiting the generation of an inrush current at power-on.
According to a first technical aspect of this invention, there is provided a driving system 1 connected to a DC power source 3 which supplies a predetermined output voltage, for driving at least one load 2, having a switch 4 for switching on and off of the supply of the voltage outputted from the power source to the load, a pulse generator 5a for generating a pulse for controlling the switch, an integrator 10 connected between the switch and the pulse generator for selectively integrating the current supplied to the load, and a current detector 12 for detecting the current supplied to the load, the current detector having first and second states, and outputting the first state when the current supplied to the load is greater than a predetermined value, or otherwise outputting the second state, wherein, the integrator operates for integration when the output from the current detector is the first state, and does not operate for integration when the output from the current detector is the second state.
According to a second technical aspect of this invention, there is provided A method of driving at least one load 2 using a DC power source 3 supplying a predetermined output voltage, comprising (1) converting a voltage outputted from the power source to a pulse voltage for output, (2) detecting a current supplied to the load and comparing the detected current with a predetermined current value, and (3) selectively integrating the pulse voltage based on the comparison result, wherein, the integration is performed to the pulse voltage when the detected current is greater than the predetermined current value, and otherwise is not performed.